XIII 



PHYLUM CHORD AT A 



35 



entb 



The zooids of the dorsal stolon consist of two sets, differing 

 from one another in position and in future history the lateral 

 zooids and the median zooids. The lateral zooids serve solely to 

 carry on the nourishment and respiration of the colony, and do 

 not undergo any further development. The median zooids, on 

 the other hand, become detached, and each develops a ventral 

 stolon. On this are found buds which have either migrated with 

 the rest from the ventral stolon 

 of the parent or have become de- 

 veloped in situ. Each of these buds 

 develops into a sexual Doliolum. 



The succession of stages in the 

 life-history of Doliolum thus briefly 

 sketched will be seen to succeed 

 one another in the following 

 order : (1) sexual form ; (2) tailed 

 larva developed sexually from (1) ; 

 (3) first asexual form the direct 

 outcome of (2) ; (4) second asexual 

 form developed on the dorsal stolon 

 of (3) from buds originating on 

 the ventral stolon ; (5) the young 

 of the sexual form (1) which are 

 developed on the stolon of 4 

 from buds which were either 

 formed there, or derived originally 

 from the ventral stolon of 3. 



Salpa, like Doliolum, presents 

 a remarkable alternation of genera- 

 tions. In the sexual form, which 

 has already been described, only 

 one ovum becomes developed. The 

 testis becomes mature later than 

 the ovum, and the latter is impreg- 

 nated by sperms from the testis 

 of an individual of an older chain. 

 The development is direct and 

 takes place Avithin the body of the 

 parent, the embryo as it grows 



projecting into the branchial cavity. The nourishment of the 

 developing embryo (Fig. 695) is effected by the formation of a 

 structure the placenta through which a close union is brought 

 about between the vascular system of the parent and that of the 

 embryo. The placenta of Salpa is partly formed from follicle-cells 

 arid ectoderm cells of the embryo, partly from the cells of the wall of 

 the oviduct. Segmentation is complete. The study of the earlier 

 stages is complicated by the very remarkable and unusual circum- 



D 2 



dors.slol 



lal.bds 



me.d.bds 



FIG. 694. Doliolum, dorsal view of 

 the posterior part of the body of an 

 asexual zooid showing the course 

 taken by the buds (emb.) over the sur- 

 face from the ventral stolon (vent, 

 stol.) to the dorsal (dors, stol.) and 

 their growth on the latter, lat. bds. 

 lateral-buds ; med. Ids. median buds ; 

 peric. pericardium. (After Barrois ) 



